pH‐Regulated Ring‐Opening/Closing Transformation in Polyoxometalates Enables High‐Performance Nonlinear Optical Crystals

Inorganic polyoxometalate (POM) crystals are a promising class of materials with considerable potential in modern laser science and technology. However, the limited understanding of POM formation mechanisms and the unclear structure-property relationship between POMs and nonlinear optics (NLO) have hindered the rational design and controllable synthesis of high-performance inorganic POM-based NLO crystals. Herein, we report the successful synthesis of two novel polar heteropolymolybdates, (NH₄)₄[Mo₅O₁₅(SeO₃)₂]·2H₂O (NMSeO-5) and (NH₄)₈[Mo₆O₁₈(SeO₃)₄]·5H₂O (NMSeO-6). Supported by theoretical calculations and experimental verification, we demonstrate, for the first time, pH-regulated ring-opening and ring-closing transformations in an inorganic POM system and propose a mechanistic pathway for their formation. Specifically, NMSeO-5, containing highly symmetrical Strandberg-type [Mo₅O₁₅(SeO₃)₂]^2- clusters, undergoes structural transformation under strongly acidic conditions (H₂SeO₃) into NMSeO-6, which features a lower-symmetry, Reuleaux triangular [Mo₆O₁₈(SeO₃)₄]^8- cluster. Remarkably, this transformation results in a dramatic enhancement of the second-harmonic generation response from 0.07 × KDP (NMSeO-5) to 10 × KDP (NMSeO-6). This work provides not only mechanistic insight into POM structural evolution but also establishes a paradigm for the design of next-generation, high-performance inorganic POM-based NLO materials.